Known plated steel sheets that exhibit excellent corrosion resistance include hot-dip Zn alloy-plated steel sheets each having a hot-dip Zn alloy plating layer containing Al and Mg formed on a surface of a base steel sheet. The plating layer of the hot-dip Zn alloy-plated steel sheet has a composition of, for example, 4.0 to 15.0% by mass of Al, 1.0 to 4.0% by mass of Mg, 0.002 to 0.1% by mass of Ti, 0.001 to 0.045% by mass of B with the balance of Zn and unavoidable impurities. Such a hot-dip Zn alloy-plated steel sheet has a plating layer made of a metal structure in which both a [primary crystal Al phase] and a [Zn single phase] are present in a matrix of an [Al/Zn/Zn2Mg ternary eutectic structure], and shows corrosion resistance and surface appearance sufficiently good as an industrial product.
The hot-dip Zn alloy-plated steel sheet can be continuously produced by the following method. First, a base steel sheet (steel strip) having passed through a furnace is immersed in a hot-dip Zn alloy plating bath containing Al and Mg, and thereafter, the amount of molten metal coating the surface of the base steel sheet is adjusted to a predetermined amount by, for example, passing the base steel sheet through a gas wiping apparatus. Subsequently, the steel strip coated with the predetermined amount of molten metal is passed through an air jet cooler and a mist cooling region, so that the molten metal can be cooled to form a hot-dip Zn alloy plating layer. Furthermore, the steel strip having the hot-dip Zn alloy plating layer formed thereon is passed through a water quenching zone to contact with cooling water, whereby a hot-dip Zn alloy-plated steel sheet is obtained.
In the hot-dip Zn alloy-plated steel sheet thus produced, however, the surface of the plating layer is partly blackened over time in some cases. The blackening of the plating layer surface occurs 2 to 3 days after the production at the earliest, and may occur in 4 to 7 days after production depending upon the production conditions. Thus, the appearance of the hot-dip Zn alloy-plated steel sheet is spoiled.
As a method for preventing the blackening, a method has been proposed in which the surface temperature of a plating layer in a water quenching zone is adjusted (see, for example, PTL 1). According to the invention disclosed in PTL 1, the blackening of the plating layer surface is prevented by setting the surface temperature of the plating layer to be less than 105° C. when contacting the plating layer with cooling water in a water quenching zone. Instead of setting the surface temperature of the plating layer to be less than 105° C., the blackening of the plating layer surface can be prevented also by adding a readily oxidizable element (rare earth element, Y, Zr, or Si) to a plating bath and setting the surface temperature of the plating layer to 105 to 300° C.